In the field of ophthalmology, myopia (near-sightedness) is the most common optical refractive error. In recent years this condition has been corrected through procedures such as radial keratotomy (RK). In this surgery, a series of radial incisions of precise depth are placed in the cornea, effecting a change in its curvature and hence its refractive power. The length and number of incisions required in an RK operation varies according to the degree of optical correction desired, and the depth of the incision is critical to the satisfactory outcome of the surgery. Inadequate depth will result in under-correction, whereas excessive depth will result in over-correction or perforation of the cornea and potentially serious harm to the eye. Accurate measurement of the corneal thickness is therefore essential to the safe and successful performance of RK surgery.
Instruments that measure corneal thickness are referred to as pachymeters. Early pachymeters were purely optical devices and have been shown to be imprecise. Ultrasonic technology is now routinely used to measure corneal thickness. All current instruments employ an ultrasonic probe that contacts the eye and which is attached via a cable to a desk unit which is about the size of an oscilloscope. Some units have a small oscilloscope screen that displays the transducer tracings and other more recent units simply display a digital readout of the corneal thickness on the device. The speed at which the echoes are sampled determines the accuracy with which the thickness measurement can be made. CMOS digital devices cannot reliably run an 11-bit synchronous counter system at 40 MHz using current semiconductor technology. It would therefore be useful to devise a CMOS based system that sampled ultrasonic echoes at frequencies in the 40 MHz range in order to achieve greater sensitivity, resolution and accuracy of measurement.
The most commonly performed operation in ophthalmology is cataract surgery. A cataract is an opacification of the biological lens inside the eye. In cataract extraction, one of several techniques is used to remove the opacified lens material. Once removed, it is possible to implant an artificial lens in order to restore optical integrity to the eye, eliminating the need for thick cataract glasses or contact lenses.
When cataract surgery is performed, several measurements are necessary in order to calculate the precise power of the intraocular lens (IOL) to be implanted. The important variables are corneal curvature (keratometry), size of the eye (axial length), and knowledge of where inside the eye the IOL is to be implanted (anterior or posterior chamber). Measurement of axial length is performed ultrasonically by a device referred to as a biometric ruler. Through such an instrument, the major internal structures of the eye can be imaged and their dimensions measured. Of importance in the measurement of true axial length is the distance from the cornea to the retina along the visual axis of the eye. The early biometric rulers employed a piezoelectric transducer, in a hand-held probe attached via a cable to an oscilloscope. It was necessary for the individual performing the examination to visually assess the oscilloscope signals and identify an axial scan. Second generation instruments still require the examiner to detect the pattern consistent with an axial scan. Then, electronics are used to calculate and display the desired measurement in millimeters. It would be desirable to have a portable, digital, ultrasonic biometric ruler for displaying the axial length of the eye, using microprocessor technology to assess the echo waveforms and displaying digitally a readout representing as accurately as possible the true axial length of the eye.